This invention relates to a method for coating various liquid coating solutions onto electrified webs for use in the manufacture of photographic film materials, photographic paper, photographic printing materials, magnetic recording materials such as magnetic recording tape, adhesive tape, and information recording paper such as pressure-sensitive paper or thermal paper.
Methods for coating webs are widely known where the surface of a continuously running web is electrified by some electrostatically charging device before a coating solution is applied, which is used in particular for realizing high-speed coating. In the coating methods using such electrified webs, a uniform charge distribution on the web is required. It is, however, not easy to give the charge-uniformity, especially in a lateral edge area of the web. Electrifying a web by a corona-discharging does not give a sufficient charging on the web in the edge area, which causes the coating to be unstable in that area.
To avoid such non-uniformity of charging of the web, Japanese patent No. 2,747,837 shows a method using a combination of grounding a web-transporting roller coated with ceramics by 0.3 to 0.5 mm thickness and a corona-discharging electrode having a width more than that of the web to extend it more than 10 mm in both sides. This method improves non-uniformity of charge for a moment, but still not enough at the time of actual coating. Japanese patent No. 2,835,659 presents another method where the web is electrified 0.5 to 2.0 KV in the surface potential after heated up to 35 to 45xc2x0 C., but still non-uniformity remains in the edge area.
An object of the invention is to provide a method to obtain and keep a uniform charge distribution on a web that is to be subsequently coated in order to establish stable high-speed coating.
It was identified after some research that non-uniform charging is caused by not only non-uniformity in electrifying the web but also by leakage of charge from the web to transporting rollers at lateral edge areas of the web while the web is transported by contacting the rollers. An electrostatic charge on the electrified web leaks from one side surface when the one side contacts the roller. The amount of leakage depends on a surface resistivity of the web. That is, the lower the resistivity of the surface, the more the leakage from the surface. The leakage happens from a back surface (a surface usually not to be coated) in contact with the rollers, which in turn causes leakage from a front surface area (a surface normally to be coated).
Generally smaller surface resistivity is given to a back surface than to a front surface in order to avoid accumulation of electrostatic charge on the back surface which attracts various type of dust in the air. If some dust is attracted to the web and transferred to the surface of coating roller (web backing roller at a coating station), this causes coating defects and other defects. Thus, relatively lower surface resistivity of the back surface necessary to avoid dust related defects, leads to charge leakage from front surface, which tends to result in non-uniformity of charge on the front surface.
Based on the above, the following methods are provided. A method for coating a web comprises the steps of preparing a web of which at least one of both edge areas of the back surface has higher surface resistivity than a central area, advancing the web to pass by a coating station, electrifying the web upstream from the coating station; and coating the electrified web at the coating station. One of the ways to provide the web with at least one back surface edge area having higher surface resistivity than the central area is to apply a subbing layer to the web so as to leave the edge area unapplied.